Two things that will help you out immensely when shooting with your 20D on a tripod...
1) Either do as Bob suggests and use manual focus, or set Custum Function 4 to allow you to use the rear * button to set focus. Then the camera won't refocus on the wrong thing when you press the shutter button/start the self timer.
2) Don't worry about setting AE on a mid tone. With digital the exposure is much more about the histogram and not blowing out the highlights. Read this article for more info:Expose to the Right
Hope this helps!